1. Field of the Invention
This invention relates generally to organic light emitting diodes, solar or photovoltaic (PV) cells and, more particularly, to electrode structures for such solar devices that provide improved manufacturing and operating performance.
2. Technical Considerations
An organic light emitting diode (OLED) is a light-emitting device having an emissive electroluminescent layer incorporating organic compounds. The organic compounds emit light in response to an electric current. Typically, an emissive layer of organic semiconductor material is situated between two electrodes (an anode and a cathode). In many conventional OLEDs, the cathode is typically an opaque metal layer and the anode is formed by a transparent conductive oxide (TCO) layer. The anode is transparent to allow light to exit the OLED. When electric current is passed between the anode and the cathode, the organic material emits light. OLEDs are used in numerous applications, such as television screens, computer monitors, mobile phones, PDAs, watches, lighting, and various other electronic devices.
Photovoltaic solar cells are in principle counterparts to light emitting diodes. Here, a semiconductor device absorbs the energy of light (photons) and converts that energy into electricity. Similar to OLEDs, the solar cell typically incorporates a TCO electrode.
In both OLEDs and solar cells, the TCO electrode should have certain characteristics. For example, the TCO electrode should have a high visible light transmission. Also, the TCO electrode should have a low sheet resistance (high conductivity). TCO conductivity is typically achieved by using an oxide coating doped with an electrically conductive material. The higher the dopant level, the lower the sheet resistance. However, increasing the dopant level can reduce the visible light transmission.
The TCO is typically a crystalline material because crystalline materials tend to be more conductive than amorphous materials and also require a lower “turn on” voltage than amorphous materials. However, a problem with crystalline materials is that their surface roughness is higher than that of amorphous materials due to their crystalline structure. If this surface roughness is too high, the crystals of a crystalline TCO anode can extend through the other layers of the OLED device and may contact the cathode, causing an electrical short.
Therefore, it would be advantageous to provide an electrode structure for use with OLEDs or solar cells that helps reduce the possibility of electrical shorts and/or reduces the turn on voltage for the device and/or maintains a desired level of electrical conductivity.